We are interested in transcription factors that function in the regulation of cell fate determination during development. Our model system is the nematode C. elegans (a non-parasitic worm) that is widely used for developmental studies because of its small size, ease of culture in the laboratory, simple anatomy, rapid proliferation, and genetics. We are currently interested in several transcription factors that have been identified in other systems as important for mesoderm patterning and muscle formation. We have shown that the C. elegans MyoD and Twist transcription factors are important for the formation and patterning of post-embryonic mesodermal cells including muscle. By studying the phenotypes that result from mutations in these genes we are beginning to define their exact roles in regulating the development of specific subsets of muscle cells in C. elegans. As part of our efforts to understand cellular differentiation, we have characterized three genes that function in the G1 phase of the cell cycle. Cells must coordinate cell cycle exit with differentiation and G1 cell cycle factors are important for this process. We have shown that cyclin E is essential for all cell cycle during development whereas cyclin D functions only post-embryonically to regulate the cell cycle. This understanding of the basic cell cycle machinery in C. elegans allows us to put the action of transcription factors into developmental context. It also opens the way to manipulating cell cycle regulation to study the interactions between cell cycle regulation and differentiation.